The present invention generally relates to reading of documents for image processing and more particularly to a system and a method for detecting a background level of a document to be processed by an image processing system such as facsimile.
In a facsimile and the like, a peak-hold circuit is used for reading and storing the background level of a document to be processed. Thus, in a so-called half-tone mode used for processing a document carrying a half-tone image such as photograph and the like, the background level of the document is detected by detecting a reference level or white level of a white marginal part of a platen roller, or by detecting the white level of a white press plate or a white head part of the document. The background level thus detected is then stored in a capacitor of a peak-hold circuit.
When reading a document such as manuscript and the like which carries an image consisted of a fully black or dark colored part and a fully white or light colored part without any intermediate tone, on the other hand, a discharging circuit is combined with the peak-hold circuit so as to follow the possible change of the background level of the document by discharging the capacitor with a predetermined time constant. Such a change of the background level may occur, for example, when a photograph or other image is pasted on a part of a manuscript.
In the foregoing method in which the background level is stored in the capacitor of the peak-hold circuit, however, there is a problem in that the background level cannot be retained for a prolonged time period because of the leak current flowing through a parasitic resistance of a semiconductor device constituting the peak-hold circuit. Thus, when the transmission of image via a facsimile transmission system is interrupted for a time period of several minutes or more, for example, the background level stored in the capacitor may be changed. When this occurs, the reading of the document thereafter cannot be achieved properly and a the quality of the transmitted image is deteriorated particularly when the image thus read is processed digitally for transmission because of the false input to the digital processing system.
When reading a half-tone image such as a photograph, on the other hand, there may be a case in which a dark image continues. Further, there arises relatively often a case in which a document containing no white part has to be processed. When this occurs, a proper background detection becomes no longer possible. When the background level detection is erroneous, the reproduced document becomes whitish or darkish and in the extremest case the image thereon becomes unintelligible.